Urea granulation process with scrubbing system

ABSTRACT

A urea granulation process with a scrubbing system may involve at least one gaseous waste stream for removal of dust and ammonia whereby the waste stream may be processed through a combination of process steps. In some examples, the process steps may involve washing the dust and ammonia laden stream with water and/or an aqueous urea solution whereby a dust-laden liquid stream and a dust-reduced stream is generated. The process steps may further involve reacting the dust-reduced stream with formaldehyde to form a stream comprising hexamethylenetetramine and urea-formaldehyde and clean off-gas. In some cases, the gas stream may be directed first through the washing step and then through the reacting step.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/399,473 filed on Nov. 6, 2014, which is a U.S. NationalStage Entry of International Patent Application No. PCT/EP2013/001292filed May 2, 2013, which in turn claims priority to European PatentApplication No. 12003585.2 filed May 8, 2012, all of which areincorporated herein by reference in their entireties.

FIELD

The present disclosure generally relates to a urea granulation processand to the apparatus suitable for operating such a process. The presentdisclosure integrates a method for reducing ammonia emissions from aurea granulation plant that is currently emitted by a conventional ureaproduction process by scrubbing the off-gas. Several advantages of thescrubbing system concern reducing an amount of ammonia in off-gas andreducing the generation of ammonium salts.

BACKGROUND

A common process for producing granules from a liquid composition isdescribed in U.S. Pat. No. 5,779,945. The focus of U.S. Pat. No.5,779,945 is the treatment and sorting of generated granules withdifferent sizes. Herein a gas/solids separating apparatus such as acyclone or a scrubber is used to separate solid material from theoff-gas stream of the apparatus. Advanced treatment of the off-gasstream is not taken into further account.

In U.S. Pat. No. 4,370,198 the off-gas of the granulation unit is sentto a dust separation cyclone followed by a continuous wet scrubber whichboth contributes to the scrubbing off said off-gas stream. The scrubbingliquid used is part of the solution or suspension to be proceeded andthe scrubbing liquid leaving the wet scrubber is fed back directly intothe granulation unit. Exemplarily, the described process can be achievedfor the production of sodium chloride, urea, saccharose or ferric oxide,respectively. Hereby the scrubbing liquor is part of the solution orsuspension to be processed and is send directly back into thegranulation unit. This process can be only achieved for dust scrubbingbut is not suitable for ammonia scrubbing.

A further example for an apparatus and a method for wet typesimultaneous cleaning and dust-removing gas treatment in a horizontalcross-flow scrubber are disclosed in EP 0853971 A1. The presentdisclosure performs the removal of pollutants and dust in a packedtower.

In a urea plant used air exiting a urea granulator that is equipped witha fluidized bed contains in addition to urea dust also ammonia. Thisammonia contamination needs to be removed before the off-gas stream canbe vented into the atmosphere.

Removing ammonia from an off-gas stream is a well-known technology.Usually the off-gas stream is treated with an acidic scrubbing solution.This scrubbing solution can be easily manufactured by adding an acidsuch as nitric acid or sulphuric acid to water. The ammonia is removedfrom the gas stream by chemical absorption and converted to thecorresponding ammonium salt. The use of nitric acid produces ammoniumnitrate (AN), and the use of sulphuric acid produces ammonium sulphate(AS) respectively. These ammonium salt-containing solutions can be usedfor the production of ammonium sulphate fertilizer or NPK fertilizer,the technology for this is state of the art.

In a urea plant, ammonium salts do not occur in the process and cannoteasily be processed at existing urea facilities. A conventional ureaproduction facility therefore has only the following options to reducegaseous ammonia emissions from the granulation plant:

-   -   to discharge the diluted ammonium salt solution to a waste water        stream,    -   to concentrate the diluted ammonium salt solution up to a        concentration which can be utilized by other plants, e.g. NPK,    -   to produce UAS (urea/ammonium sulphate) fertilizer with a high        sulphur content,    -   to produce UAN (urea/ammonium nitrate) solution.        All of these alternatives require significant investments and        changes to operating conditions or entail changes of the product        composition and characteristics. All above options result in new        products that require additional facilities for transport and        handling as well as energy utilities in expensive quantities. As        a consequence, nowadays, urea facilities are run without        efficient ammonia removal causing severe environmental problems.        Therefore, ammonia removal from a urea facility is a challenging        task that needs to be solved.

An alternative solution is described in WO 03/099721. The presentdisclosure relates to a process for removing ammonia from anammonia-containing gas stream by converting the ammonia in theammonia-containing gas stream with an organic acid into an ammoniumsalt, whereas the obtained ammonium salt is contacted, at elevatedtemperature, with peroxide. The ammonium salt is hereby converted into aNH₃, CO₂ and H₂O containing mixture in a decomposer and can readily bereprocessed in a urea synthesis unit. The peroxide is supplementary tothe common process and may relate to other negative accompaniments.Also, for the conversion of the ammonium salt into NH₃, CO₂ and H₂O aseparate decomposer in addition to the normal plant layout is required.This emerging gas stream can not be reprocessed in a granulation unitbut needs to be recycled in a urea synthesis unit.

Reductions of ammonia emissions are also described in M Potthoff,Nitrogen+Syngas, [online], July. August 2008, pages 39-41. In FIG. 1 acombined dust and acidic scrubber system is shown. The ammonia isabsorbed in the acidic scrubbing section and converted into ammoniumsulphate. The ammonium sulphate solution is added to the recycle flowgoing back to the evaporation section. In this unit it is mixed withurea melt from the urea synthesis unit. The concentrated liquor streamfrom the evaporation is conveyed into the urea granulator. Thecondensate coming out of the evaporation unit is utilised as makeup forthe combined dust/ammonia scrubbing system. With this so called AmmoniaConvert Technology ammonia in off-gas can be reduced to 30 mg/Nm³. Thetechnology without acidic scrubbing as shown in Brochure Urea, [online],12-2007, pages 1-24 reduces ammonia in off-gas only to values of around160 mg/m³.

The ammonia convert technology described in M Potthoff, Nitrogen+Syngas,[online], July. August 2008, pages 39-41 implicates still severaldisadvantages. First of all, the water balance in this system is acritical parameter. If disturbed, urea synthesis will be contaminatedwith ammonium sulphate or alternatively large amounts of waste waterneed to be treated. In addition, mixing of acidic solution withconcentrated urea melt in the evaporation unit has adverse effects ongranulation. Moreover, this technology implicates the generation oflarge amounts of condensate contaminated with ammonium sulphate thatneeds to be distributed to various scrubbers, including dust and acidicscrubbing technology. Also the remaining ammonia concentration in theoff-gas achieved with this technology is still not sufficient orsatisfactory for modern urea granulation plants.

In WO 2010/060535 A1 the ammonia convert technology described in MPotthoff, Nitrogen+Syngas, [online], July. August 2008, pages 39-41 isimproved in order to achieve ammonia concentrations in off-gas of 10mg/Nm³. WO 2010/060535 A1 teaches that a scrubber dust stage, that isconnected to process coolers, is operated through an ammonium saltsolution stream generated in a scrubber acid stage, which is connectedto the urea granulator. Therefore the scrubbing system presented in WO2010/060535 A1 represents an in itself complete closed system asdescribed in the characteristic part of claim 1 of the presentdisclosure. This technology avoids contamination of the urea meltgenerated in the urea synthesis unit by building such a complete closedscrubbing system. The disadvantage of this system is that it is verycomplex in its performance.

In U.S. Pat. No. 5,686,647 a process for preparing urea is describedwherein an amount of formaldehyde is added to an off-gas streamcontaining gaseous ammonia to form hexamethylenetetramine, which isreturned into the process before the granulation step. This formaldehydeaddition can be performed before or during a washing step with liquidurea solution whereby this washing step serves as dust scrubbing device.The disadvantage of this technology is the relatively high amount ofammonia in the off-gas of circa 90 mg/Nm³ in comparison to thetechnology presented in WO 2010/060535 A1.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in detail below with reference tothe attached drawing figures.

FIG. 1 is a schematic block diagram of an embodiment of a ureagranulation process utilizing an ammonia formaldehyde convert process,as disclosed herein.

FIG. 2 is a schematic block diagram of an embodiment of a ureagranulation process utilizing an ammonia formaldehyde convert process,including a scrubber acid stage, as disclosed herein.

FIG. 3 is a schematic block diagram of an embodiment of a ureagranulation process including a closed scrubbing system, as disclosedherein.

DETAILED DESCRIPTION

An object of the present disclosure is to provide a process thatintegrates and optimizes technology for scrubbing off-gas generated by aurea granulation process. The process eliminates problems associatedwith the use of conventional technologies as described above, and shouldbe easy to integrate into current state of the art scrubbing systems. Itis also an object of the present disclosure to provide an apparatussuitable to perform such a process.

This object and other objects are achieved by a urea granulation processwith scrubbing system including at least one gaseous waste stream forremoval of dust and ammonia whereby this waste stream is processedthrough a combination of the following process steps comprising

-   -   (a) washing the dust and ammonia laden stream 4 with water        and/or an aqueous urea solution whereby a dust-laden liquid        stream 26 and a dust-reduced stream 5 is generated, and    -   (b) reacting the dust-reduced stream 5 with formaldehyde 7 to        form a stream comprising hexamethylenetetramine and        urea-formaldehyde 8 and clean off-gas 6    -   wherein the gas stream is directed first through process        step (a) and then through process step (b).

Surprisingly the sequence of process steps in the above disclosedembodiment of a method of the present disclosure permits furtherreduction of ammonia emissions from granulation plants in comparison tothe technology described in U.S. Pat. No. 5,686,647, in which the orderof process steps is reversed. If process step (b) is done before thedust scrubbing in process step (a) the reaction ammonia-formaldehydesuffers from competition with the standard urea-formaldehyde reactionwhich would prevail in the dilute urea solution obtained in thescrubber. Therefore efficiency in this process step is lost andammonia-reduction is limited.

Hereby the urea concentration of the dust-laden liquid stream 26 is keptin a range from 35 to 60% wt, and preferably is kept in a range from 45to 55 wt and that dust laden liquid stream 26 is returned into theprocess before the granulation step.

Furthermore 70 to 90 wt % of ammonia in relation to the total ammoniacontent of the dust-reduced stream 5 is reacted tohexamethylenetetramine in the formaldehyde stage 2.

Optionally, the stream comprising hexamethylenetetramine andurea-formaldehyde 8 is returned into the process before the granulationstep. The hexamethylenetetramine comprises urea-formaldehyde solutionand therefore replaces at least part of the urea/formaldehyde solutionnormally used as granulation additive.

In a further embodiment of the current process the dust-laden liquidstream 26 is mixed with the stream comprising hexamethylenetetramine andurea-formaldehyde 8 before returning this mixture into the processbefore the granulation step.

In a further embodiment of the present disclosure an additional processstep for removing ammonia is implemented downstream of process step (b)wherein an ammonia-laden stream is brought into contact with an acid 9in liquid phase and thereby ammonia is scrubbed from that stream by thegeneration of an ammonium salt stream 10 in a scrubber acid stage 3.

The combination of these three process steps bears the advantage thatthe amount of ammonium salt generated in the scrubber acid stage 3 isgreatly reduced so that these salts do not disturb the granulationsystem or the urea synthesis system if recycled back in one of thesesystems. Also the amount of ammonia reduced by this system can beimproved.

Hereby 94 to 99.9% of ammonia in relation to the total ammonia contentof the dust- and ammonia-laden stream 4 is eliminated through thecombination of process steps (a) and (b) with a further acidictreatment.

In an embodiment of the present disclosure the acid is selected from thegroup consisting of sulphuric acid, nitric acid, phosphoric acid, citricacid, lactic acid and oxalic acid. Other acids can be used if they arenon-volatile. Preferably, sulphuric acid is used, as it is readilyavailable and in addition, it supplies sulphur which is considered to bea highly demanded nutrient.

Furthermore the ammonia salt concentration of the ammonium salt streamgenerated in the scrubber acid stage is kept <40% wt, and preferably iskept in a range from 35-40% wt.

The pH of the ammonia salt stream generated in the granulator scrubberacid stage is kept in a range from 2-6, and preferably is kept in arange from 3.5-5.0, and most preferably is kept in a range from 4.0-4.5.

In an optional embodiment a second gaseous dust- and ammonia-ladenstream 14 drawn off from product coolers 13 is generated, which streamis send through a further scrubber dust stage 15 in which the ammoniumsalt stream 10 of the further acid treatment is used to remove theammonia from this second gaseous dust- and ammonia-ladden stream 14.

In a further optional embodiment of the present disclosure the scrubbingsystem being passed is in itself a complete closed system, whereby

-   -   the ammonium salt stream 10 from the scrubber acid stage 3 is        fed into said further scrubber dust stage 15, and    -   the released solution 17 from said further scrubber dust stage        15 is send to a evaporation unit 16,    -   the vapour stream 18 from the evaporation unit 16, which        contains ammonia is given into a condenser unit 19, which        releases a liquid process condensate 20, and said liquid process        condensate 20 is given into the scrubber acid stage 3, and    -   the concentrated liquor stream 21 generated in the evaporation        unit 16, containing urea and ammonium salt, and a urea melt 22        is conveyed into the urea granulator 1.

Hereby the scrubbing system in itself is a complete closed system, andis therefore totally decoupled from urea synthesis. Therebycontaminations of the urea melt are totally avoided.

With advantage the concentration of the urea melt 22 and concentratedliquor stream 21, containing urea and ammonium salt, for the ureagranulator being kept in a range from 95 to 99.8% wt, and beingpreferably kept in a range from 96 to 97.5% wt.

Optionally a portion of urea melt 22 is fed into the evaporation unit16.

Furthermore the clean off-gas 6 is released into the atmosphere andexhibits a concentration of NH₃ in the range of 5-30 mg/Nm³, andpreferably exhibits a concentration of NH₃ being <10 mg/Nm³.

The present disclosure also comprises an apparatus with scrubbing systemsystem including at least one gaseous waste stream for the removal ofdust and ammonia comprising

-   -   a scrubber dust stage 11, in which dust is washed off from a        dust- and ammonia-ladden stream, and    -   a formaldehyde stage 2, in which part of the ammonia of the        ammonia-ladden air 4 is reacted with formaldehyde 7 to form        hexamethylenetetramine, whereby the scrubber dust stage 11 is        arranged upstream of the formaldehyde stage 2.

Furthermore an additional scrubber acid stage 3 is integrated into thescrubbing system downstream of the formaldehyde stage 2.

Optionally the urea granulation apparatus with scrubbing systemcomprises also product coolers 13, in which a second gaseousammonia-laden stream 14 is generated, and which product coolers 13 areconnected with a further scrubber dust stage 15 which is connected withmeans for conveying the ammonium salt solution stream 10 from thescrubber acid stage 3 to said further scrubber dust stage 15.

In a further embodiment of the urea granulation apparatus theapparatuses of the scrubbing system being connected in such a way that acomplete closed system of waste streams is built, comprising

-   -   means for conveying the ammonium salt stream 10 from the        scrubber acid stage 3 to the further scrubber dust stage 10, and    -   means for conveying the solution 17 from said further scrubber        dust stage 15 to an evaporation unit 16,    -   means for conveying the steam vapour 18 of the evaporation unit        16 to a condenser unit 19,    -   means for conveying the process condensate 20 from the condenser        unit 19 to the granulator scrubber acid stage 3, and    -   means for conveying urea melt 22 and a means for conveying a        concentrated liquor stream 21, containing urea and ammonium salt        into the urea granulator 1.

Furthermore the apparatus comprises means for conveying a portion ofurea melt to the evaporation unit 16.

With advantage scrubbers used in the current technology are horizontalscrubbers.

With reference now to the figures, FIG. 1 shows an urea granulator 1,which is supplied with urea melt or an aqueous urea solution 22. In theurea granulator 1 urea granules are formed in a fluidized bed, which isfluidized by an air stream 27. A dust- and ammonia-laden stream 4 isdrawn off. It is first scrubbed in the scrubber dust stage 11, whereurea dust is removed. A stream of process water or diluted urea solution12 is added to the scrubber dust stage 11 and the dust-laden stream 26is drawn-off from the scrubber dust stage 11. The dust-reduced stream 5is then sent to the formaldehyde stage 2. According to the presentdisclosure formaldehyde 7 is introduced in the formaldehyde stage 2. Ahexamethylenetetramine and formaldehyde containing stream 8 is drawn-offfrom the formaldehyde stage 2. This hexamethylenetetramine can bereturned into the granulation process before the granulation step. Theclean off-gas 6 is send into the atmosphere.

FIG. 2 includes in comparison to FIG. 1 an additional scrubber acidstage 3 downstream of the formaldehyde stage 2. The ammonia reducedstream 29 from the formaldehyde stage 2 is send into the scrubber acidstage 3 where the rest of ammonia is removed, and the clean off-gasstream 6 can be drawn off. The scrubbing solution for the scrubber acidstage 3 consists of process water and the acid 9 in liquid phase. In thegranulator scrubber acid stage 3 the acid solution reacts with ammoniaproducing an ammonium salt stream 10. This ammonium salt stream 10 canbe further processed as shown in FIG. 3 or can be drawn-off from theurea granulation system.

This inventive process allows the reduction of ammonium salts generatedin the scrubber acid stage 3 but is very effective in reducing ammoniaemissions from urea granulation plants. Ammonium salts are muchundesired because they cause severe environmental problems and causeproblems in urea granule quality if added to high concentrations to thegranulation process.

FIG. 3 includes in comparison to FIG. 2 a in itself closed scrubbingsystem including the inventive process steps. In addition to FIG. 2product coolers 13 are shown, in which the hot granules 25 produced areconveyed. Air 28 cools the final product 25. The dust-laden air stream14 is conveyed to a further scrubber dust stage 15, where the urea dustis washed out while the air is cooled down by evaporation of water inthe scrubber. The clean off-gas 23 leaving the scrubber dust stage 15 isto the atmosphere.

The resulting solution 17 from the scrubber dust stage 15, is combinedwith the dust-laden stream 26 from the granulator scrubber dust stage 11and the resulting mixture is conveyed to the evaporation unit 16, whereit is concentrated. The concentrated liquor stream 21 from theevaporation unit 16 is fed to the urea granulator 1 to integrate thegenerated ammonium salt into the granulation process. A portion of theurea melt 22 can be added to the evaporation unit 16 (not shown), inorder to keep the urea concentration and the ammonium sulphateconcentration of the concentrated liquor stream 21 in the right ratio.The steam vapour 18 drawn off from the evaporation unit 16 is conveyedto a condenser unit 19, where it is cooled by external cooling water.The liquid process condensate 20 generated during the condensation issend into the scrubber acid stage 3. To close the scrubbing cycle theammonium salt stream 10 drawn-off from the scrubber acid stage 3 is sendto the scrubber dust stage 15.

Therefore a closed circle of waste streams is formed and all wastestreams are recycled. In addition the generated ammonium salts areintegrated in the urea granulation process. Also external process waterconsumption is reduced to a minimum. Altogether, this combination ischaracterized by its environmental compatibility. Also the content ofammonium salt in the generated urea granules is reduced, which getsproblematic if sulphuric acid is used as acid 9 and the sulphur contentof the granules increase.

Example 1

In example 1 a table is shown giving some typical figures concerningammonia in the urea granulation processes state of the art as describedin Brochure Uhde, Urea, [online] 2011 compared with a formaldehydetreatment as described in U.S. Pat. No. 5,686,647 implemented before orcombined with a scrubber dust stage and the inventive technology.

In a urea granulation process with formaldehyde scrubbing aformaldehyde-containing solution is added to the ammonia-laden air orthe formaldehyde stage.

The formaldehyde-containing solution used for scrubbing is charged withhexamethylenetetramine and is partially reintroduced into the abovedescribed urea process. Basically this mixture after being brought tothe right pressure and temperature may be recycled in every phase of theprocess.

The amount of ammonia of 500 to 600 ppm by weight in the feed to thegranulation unit is more or less unavoidable as it is the result of theequilibrium formed in an upstream to the granulation unit arrangedevaporation unit, if the concentrated liquor stream generated in thisevaporation unit shall be introduced into the granulator. About 90 ppmammonia is added through biuret formation in the urea solution, which isfed into the granulator, so that in total about 590 to 690 ppm entersthe granulation unit.

About 50 ppm of this ammonia is included in the final product, wherebythe rest leaves the granulation plant with the air flow from thegranulation unit via stacks. This results in a final concentration ofapproximately 130 to 160 mg/Nm³ for the technology state of the art aspresented in Brochure Urea, [online], 2011. In the technology describedby U.S. Pat. No. 5,686,647 a final concentration of circa 86 mg/Nm³ canbe reached. If formaldehyde stage is put into practice after a dustscrubber where the urea is almost removed, as the present disclosureshown in FIG. 1 suggests, a final concentration of approximately 30mg/Nm³ ammonia is found in a combined stack. The inventive technology incombination with a following acid scrubbing stage as shown in FIG. 2 canlead to ammonia concentrations of 10 mg/Nm³ with a minor amount of acidto be used. Therefore a drastically improvement can be achieved usingthis technology.

TABLE 1 technology state of the art in comparison with presentdisclosure formaldehyde treat- formaldehyde treat- Formaldehyde treat-technology state ment as described in ment of the present ment of thepresent of the art U.S. Pat. No. 5,686,647 disclosure as showndisclosure as shown (Brochure Urea, 2011) before a scrubber dust stagein FIG. 1 in FIG. 2 Free ammonia from □ 500 to 600 ppm wt. evaporationunit Ammonia from □ 90 ppm wt. biuret formation Total free ammonia □ 590to 690 ppm wt. at granulator inlet Free ammonia in □ 50 ppm wt. finalproduct Free ammonia □ 540 to 640 ppm wt. released (based on ureasolution) Ammonia - none yes yes yes formaldhyde stage Dose Formaldehyde4 4 4 as UFC via 7 kg/ton Typical ammonia □ 130 to 160 □ 86 □ 30 □ 10concentration in mg/Nm³ mg/Nm³ mg/Nm³ mg/Nm³ combined stack □ 0.6 to 0.7□ 0.40 □ 0.14 □ 0.05 kg/ton_(product) kg/ton_(product) kg/ton_(product)kg/ton_(product) Formaldehyde 45.00% 75.00% 75.00% efficiency Ammonium≅0.35 sulphate produced kg/ton_(product)

The efficiency of formaldehyde to abate ammonia is strongly reduced toonly 45% if the process is done before the dust scrubbing. If it is donein combination with the dust scrubbing the formaldehyde efficiency is75%. The reaction ammonia-formaldehyde suffers from competition with thestandard urea-formaldehyde reaction which would prevail in the diluteurea solution obtained in the scrubber. Therefore the change in thesequence of process steps of the present disclosure in relation to theteaching of U.S. Pat. No. 5,686,647 has an enormous positive effect inrespect to the ammonia content in off-gas. The combination shown in FIG.2 of a scrubber acid stage downstream of the formaldehyde stage has theadvantage that the ammonium salt stream generated has a very lowammonium salt concentration if compared with the technology state of theart of WO2010060535A1 (table 2) in which a formaldehyde stage ismissing. Therefore this ammonium salt stream can be exported from thegranulation system or can be further processed as shown in FIG. 3.

Example 2

In example 2 a table is shown giving some typical figures concerningammonia in the urea granulation processes state of the art as describedin WO2010060535A1, in which the ammonium salt stream generated isreintroduced into the granulation process, whereby a in itself completeclosed system of scrubbing streams is built, compared with the inventiveclosed scrubber technology as shown in FIG. 3.

In a urea granulation process with a scrubber system according to FIG. 3a formaldehyde-containing solution is added via 7 to the formaldehydestage 2.

The formaldehyde-containing solution used for scrubbing in formaldehydestage 2 is charged with hexamethylenetetramine and is partiallyreintroduced into the standard urea process. Basically this mixtureafter being brought to the right pressure and temperature may berecycled in every phase of the process.

TABLE 2 technology state of the art in comparison with presentdisclosure as shown in FIG. 3: technology state Ammonia convertInventive of the art technology technology (Brochure Urea, 2011)WO2010060535A1 (FIG. 3) Free ammonia from □ 500 to 600 ppm wt.evaporation section Ammonia from □ 90 ppm wt. biuret formation Totalfree ammonia □ 590 to 690 ppm wt. at granulator inlet Free ammonia in □50 ppm wt. final product Free ammonia □ 540 to 640 ppm wt. released(based on urea solution) Ammonia - none none yes formaldhyde stage DoseFormaldehyde 4 as UFC via 7 kg/ton Formaldehyde 75.00% efficiency Acidscrubber none yes yes stage Typical ammonia ≈130 to 160 ≈10 ≈10concentration in mg/Nm³ mg/Nm³ mg/Nm³ combined stack ≅0.6 to 0.7 ≅0.05≅0.05 kg/ton_(product) kg/ton_(product) kg/ton_(product) Sulphuric acid≅2.0 ≅0.27 consumption kg/ton_(product) kg/ton_(product) Ammoniumsulphate ≅2.3 ≅0.35 produced kg/ton_(product) kg/ton_(product)

Thus, a solution is produced which shows ammonia concentrations inoff-gas that are comparable to those reached with the technologydescribed in WO2010060535A1. But in addition a very low ammonium saltconcentration, which is approx. 8 times less then the technologydescribed in WO2010060535A1 is produced. Also the sulphuric acidconsumption is 8 times lower which is a significant cost reduction.There is no significant change to the product specification and qualityby the addition of these small amounts of ammonium salts. The N contentof the urea product stays above 46% N, so that the product is still atypical urea fertilizer.

Some example advantages of the proposed process include the following:

-   -   significant low ammonia emissions to the environment;    -   urea granule with very low ammonium salt concentration;    -   cost benefits are achieved by reducing the ammonia and acid        consumption;    -   a simple way is used to process ammonia-laden gas streams in        existing urea granulation plants;    -   a proven and low-cost technical process is used to remove        ammonia from the off-gas streams from the urea granulation plant        with fluidized bed granulation;    -   as the recovered ammonia is included in the product the urea        production is increased, leading to a significant economic        benefit; and    -   a typical urea fertilizer grade product is produced.

What is claimed is:
 1. A urea granulation system comprising: a ureagranulator configured to generate granulated urea product and a firstdust and ammonia-laden gaseous waste stream; a first scrubber dust stagedisposed downstream of, and in communication with, said urea granulatoralong a flow path of the first gaseous waste stream, said scrubber duststage being configured to remove dust from the first dust andammonia-laden gaseous waste stream, and convey the first gaseous wastestream downstream for further processing; and a formaldehyde stagedisposed downstream of, and in communication with, said scrubber duststage along a flow path of the first gaseous waste stream, saidformaldehyde stage being configured to receive the first gaseous wastestream and react a portion of the ammonia contained in the first gaseouswaste stream with formaldehyde, so as to form hexamethylenetetramine. 2.The urea granulation system of claim 1 further comprising a scrubberacid stage disposed downstream of, and in communication with, saidformaldehyde stage along a flow path of the first gaseous waste stream,said scrubber acid stage being configured to remove additional ammoniafrom said first gaseous waste stream and generate an ammonium salt wastestream.
 3. The urea granulation system of claim 2 further comprising: atleast one product cooler in communication with said urea granulator,said product cooler being configured to cool granulated urea productconveyed to it from said urea granulator and generate a second dust andammonia-laden gaseous waste stream; and a second scrubber dust stagedisposed downstream of, and in communication with, said product cooleralong a flow path of the second dust and ammonia-laden gaseous wastestream, said second scrubber dust stage being configured to use theammonium salt waste stream from said scrubber acid stage to removeammonia from the second dust and ammonia-laden gaseous waste stream. 4.The urea granulation system of claim 1 further comprising: anevaporation unit in communication with at least the first scrubber duststage and configured to accept at least a dust laden liquid stream fromthe first scrubber dust stage and generate a concentrated liquor streamcontaining urea and ammonia salt; and a conveyor in communication withsaid evaporation unit and configured to convey a urea melt to saidevaporation unit.